1. Field of the Invention
The present invention relates to a photo-lithography apparatus used for fabricating LSI devices of high integration density, and more particularly to a photo-lithography apparatus comprising a system for automatically detecting and correcting off-telecentricity of the photo-lithography apparatus and a method for using the same.
2. Description of the Related Art
A photo-lithography apparatus plays an important role in mass production of LSI devices. At present, semiconductor devices having about half-micron patterns can be produced using the photo-lithography apparatus (stepper) using i-line illumination (365 nm wavelength light from mercury lamp) and magnification factor of 5:1.
In order to further improve resolution toward fabrication of sub-half-micron devices, new photo-lithography technologies are under investigation, among which phase shift mask and modified illumination (oblique illumination) technologies play important roles. These two technologies utilize 180.degree. phase difference between the lights passing through closely adjacent slits. In the prior art, an effective illumination source uses only one aperture stop, however, the above new technologies require different aperture stops depending on an illumination mode. In using the phase shift mask, the effective illumination source requires an aperture stop having an opening of a smaller diameter than that of the conventional aperture stop. On the contrary, the oblique illumination mode needs an aperture stop which prevents transmission of light at the central portion of the aperture stop, in other words, an aperture stop which has an annular opening or four openings arranged symmetrically with respect to the center of the aperture stop.
In manufacturing LSI or ULSI devices, each semiconductor device needs a lot of mask processes till the end of fabrication, and each mask process needs different precision in photo-lithography. Therefore, the aperture stop is changed frequently depending on the mask (reticle) to be illuminated. Usually, a plurality of aperture stops are arranged on a disk and a single aperture stop is selected by rotation of the disk in the photo-lithography apparatus.
The change of the aperture stop as described above, together with the reticle change, introduces a minor shift in an axial direction or in a lateral (transversal) direction of an optical system of the photo-lithography apparatus. An ideal optical system of photo-lithography apparatus is often called an optically telecentric system. The "telecentric" means that light passing through an image plane of the optical system in the apparatus is parallel to an optical axis (Z-axis), therefore, coordinates in xy plane for a focused pattern are constant when z coordinate is changed within a depth of focus, and magnification (in this case, reduction) is also constant within a depth of focus. The change of aperture stops is liable to make the optical system of the photo-lithography apparatus off-telecentric.
Before describing the existing method of detecting and correcting off-telecentricity of photo-lithography apparatus, an outline of the photo-lithography apparatus is first described. FIG. 1 is a schematic diagram of the existing photo-lithography apparatus. An illumination system comprises an effective illumination source 1 and a condenser lens 2. The effective illumination source 1 further comprises a mercury lamp 21, an input lens 22, a flyeye lens 23, and an aperture stop disk 24. UV light radiated from the mercury lamp 21 is refracted by the input lens 22 to the parallel light in passing therethrough, and the light is next input to the flyeye lens 23. The flyeye lens 23 is composed of a plurality of square cross-sectional rod lenses, and has a function of modifying the input light of non-uniform intensity distribution to the light of substantially uniform intensity distribution at an object plane (reticle 6) and an image plane (substrate 7).
A top view of the aperture stop disk 24 is shown in FIG. 2. The aperture stop disk 24 comprises, for example, five aperture stops 24a through 24e. Three aperture stops 24a, 24b, and 24c have circular holes of different diameters, aperture stop 24d has an annular opening, and aperture stop 24e has four holes symmetrically arranged. The aperture stops 24d, 24e are used for the oblique illumination mode.
In FIG. 1, the light passing through the selected one of aperture stops 24 is focused by the condenser lens 2 and illuminates reticle 6 on which patterns of a chromium layer are formed. The shaped light with the reticle patterns further passes through a projection lens 3 and is focused on a substrate 7 disposed on an XYZ stage 4. The XYZ stage 4 is movable in each of X, Y, and Z axis-directions by a drive mechanism 10, and a position of the XYZ stage can be precisely measured by a laser interference meter 11 and precise position data of coordinates (x, y, z) can be obtained.
In order to obtain an off-telecentricity amount, a reticle 6 having specific patterns formed thereon is used. The specific patterns on the reticle are composed or a main pattern of a square shape and an auxiliary pattern of a square-shaped slit pattern, and these patterns may be separated, but separation distances between two patterns should be known precisely. The reticle 6 is mounted on a support 28 and movable in X and Y axis-directions by a drive mechanism 29.
In FIG. 3, a semiconductor wafer 7 with a resist layer formed thereon is disposed on the XYZ stage 4, and the XYZ stage is moved such that a reduced image 26 of the main pattern is focused at the center of the wafer 7 as shown in FIG. 3, and the resist on the wafer 7 is once exposed. Next, the XYZ stage 4 is moved by the drive mechanism 10 in the Z axis-direction (axial direction) by .DELTA.z within the depth of focus, and the reticle 6 is moved by the drive mechanism 29 in the X and Y axis-directions by the known separation distances such that the center of the auxiliary pattern image 27 is theoretically coincident with the center of the main pattern image 26. However, in actual case, the center of the auxiliary pattern image 27 is slightly shifted from that of the main pattern image 26 as shown in FIG. 3. The resist layer on the wafer is again exposed under these conditions.
The wafer is taken out from the photo-lithography apparatus, and the resist layer on the wafer 7 is developed. In FIG. 4, the resist layer having main pattern 26 and auxiliary pattern 27 is formed on the wafer. Coordinates of X.sub.1, X.sub.2 ; and Y.sub.1, Y.sub.2 for the main pattern 26, and coordinates of side positions X.sub.3, X.sub.4 ; and Y.sub.3, Y.sub.4 for the auxiliary pattern 27 are precisely measured.
When shift distances of the center of the auxiliary pattern 27 in the X and Y axis-directions from the center of the main pattern 26 are expressed as .DELTA.X and .DELTA.Y, then the following results are obtained. EQU [(X.sub.4 --X.sub.3)--(X.sub.2 --X.sub.1)].times.1/2=.DELTA.X EQU [(Y.sub.4 --Y.sub.3)--(Y.sub.2 --Y.sub.1)].times.1/2=.DELTA.Y
The off-telecentricity amounts of the apparatus in the X and Y axis-directions are respectively given by ratios .DELTA.X/.DELTA.Z and .DELTA.Y/.DELTA.z.
When the off-telecentricity amounts are thus detected, the effective illumination source 1 is manually adjusted in the X and Y axis-directions corresponding to the detected off-telecentricity amounts.
The off-telecentricity described above is mainly caused by a shift of the effective illumination source in the X and Y axis-directions (for convenience, this called illumination off-telecentricity).
When a shift of the effective illumination source occurs in the Z axis-direction (for convenience, this is called magnification off-telecentricity), this causes a change in magnification when a wafer position moves in the Z axis-direction. To detect an amount of the magnification off-telecentricity, the similar method can be applied, but the details thereof are omitted.